Balance wheel for electric watch



May 17, 1960 P. E. BIEMILLER ETAL BALANCE WHEEL FOR ELECTRIC WATCH Filed Aug. 12, 1957 I N VENT 0R8 Phili Batman afl John A. Vamliorzz ATTORNEY U ited State Patent BALANCE WHEEL FOR ELECTRIC WATCH Philip E. Biemiller, Landisville, and John A. Van Horn,

Lancaster, Pa., assignors to Hamilton Watch Company, Lancaster, Pa., a corporation of Pennsylvania Application August 12, 1957, Serial No. 677,654

6 Claims. (Cl.,5828) This application relates to a balance wheel for a time piece and more particularly relates to a temperature compensated non-magnetic balance wheel for use in an electric watch wherein the balance wheel serves as the frequency controlling element and is periodically energized during passage through a magnetic field.

It is well known that the period of oscillation of a system consisting of a spiral spring and a balance varies, among other things, in direct ratio-to the square root of the inertia of the balance and in reverse ratio to the square root of the Youngs modulus of the spiral spring. Since the Youngs modulus decreases when the temperature rises, it has been necessary, in order to maintain the period .of oscillation constant, to provide for diminishing the inertia of the balance when the temperature rises. For a long time this was conventionally achieved through the use of a bi-metallic balance, the rim of which is a bi-metallic strip of brass and steel.

Thisparticular attempt at achieving a fixed period of oscillation independent of temperaturechanges was to a large extent abandoned on the discovery of the steel alloy now known as Elinvar. The Youngs modulus of this alloy and subsequent improvements thereof varies little, if at all, with temperature variations, at least between the limits of temperature to which watches are normally subjected.

In electric watches of the type shown, for instance, in assignees co-pending application Serial No. 409,934, filed February 12, 1954, now Patent No. 2,888,797, wherein the balance wheel carries a coil cooperating with a magnetic circuit which produces stray magnetism, it has been found that springs made of Elinvar and similar alloys are generally unsatisfactory because this metal is magnetic and in a magnetic field no longer supplies a restoring torque which follows Hookes law. Also, the temperature compensation is altered in the field. The magnetic materials in the balance wheel are found objectionable for the same reason. It is accordingly desirable to use spring and balance wheel materials which are non-magnetic, and we have found beryllium copper to be quite satisfactory for this purpose. 0n the other hand, when this material is used, temperature compensation is again necessary.

In addition to providing the necessary temperature compensation, the balance wheel must serve as a support for the electric coil which provides the energization for the watch, and means must be provided for poising the wheel after this coil has been added. It has been found that this can be accomplished in a very satisfactory advantageous manner through the use of a generally X-shaped skeletal balance wheel which carries the coil mounted within the arms of one-half of the X, and which carries a synthetic resin member in the arms of the other half of the X. Suitable soft metal weights are provided in the resin member, and these may be drilled or filed to provide the necessary poise.

It is accordingly a primary object of the present invention to provide an improved temperature compensated non-magnetic balance wheel for use with a non-magnetic spiral spring in electric watches wherein the balance wheel serves as the frequency regulating element and also as the means for transforming electrical energy into mechanical energy and wherein some stray magnetic fields are encountered.

It is another object of this invention to providean improved non-magnetic temperature compensated balance wheel for use with a non-magnetic spiral spring in electric watches wherein the balance wheel has a coil mounted thereon in one sector thereof and has a poising means mounted in an opposite sector.

It is another object of this invention to provide a nonmagnetic balance wheel having a multi-turn coil mounted thereon and having non-magnetic bi-metallic temperature compensating arms.

It is a still further object of the invention to provide an improved non-magnetic temperature compensated balance wheel of the foregoing type wherein the balance wheel comprises a generally X-shaped frame carrying a coil in one-half of the X and carrying a synthetic resin insert in the other half of the X for poising the balance wheel. These and further objects and advantages of the invention will become more apparent upon reference to the following specification and claims and appended drawing which shows a plan view of a balance wheel constructed according to the invention.

Referring to the single figure of the drawing, there is shown a balance wheel having a hub 10 to which four radial arms 12, 14, 16 and 18 are attached. The arms 12, 14, 16 and 18 form a generally X-shaped frame which is completed by a first arcuate rim section 20 at tached to arms 12 and 14, and a second arcuate rim section 22 attached to arms 16 and 18. The hub 10, arms 12, 14, 16 and 18 and arcuate rim sections 20 and 22 may all be integrally cast. The metal used should be non-magnetic and is preferably beryllium copper.

Mounted in the space between arms 12 and 14 and arcuate rim section 20 is a sector-shaped coil 24. As described in assignees co-pending application Serial No. 408,695 filed February 8, 1954, this coil maybe molded into a sector-shaped piece of synthetic resin 26, which may be cemented or otherwise secured to the arms 12 and 14 and the rim section 20. As an alternative, the resin may be cast about the coil within this space so as to partially surround the legs 12 and 14 and rim section 20 to thereby secure the coil in position. The inner end 28 of the coil may be connected to a nut 30 cast into the resin sector 26, while the outer end 32 of the coil may contact the arm 14. This type of coil contact arrangement is discussed in further detail in assignees copending application Serial No. 631,904, filed December 31, 1956.

The space between legs 16 and 18 and arcuate rim sec tion 22 is filled with a synthetic resin member 34 which has a series of soft metal studs 36 cast therein for purposes of poising the balance and counter-balancing the coil assembly. The studs 36 may extend above the surface of the resin sector 34 so that they may be filed to poise the wheel. These studs may be conveniently made of gold or other soft heavy metal which is easily filed or drilled for poising.

Any suitable casting resin may be used for encapsulating the coil 24 and for the sector 34. Either epoxies or polyesters may be used. As an example of epoxies, the following commercial resins are satisfactory: Scotchcast" marketed by Minnesota Mining, Marblette 622B marketed by the Marblette Corporation, Epon marketed by Shell Chemical Corporation. Examples of polyesters are the following commercial products, the EP line of resins manufactured by Electronic Plastics Corporation and Stycast 4030 CM marketed by Emerson and Cummings.

Temperature compensation for the balance wheel is provided by means of a pair of composite arcuate arms 38 and 40. The arm 38 consists of an inner arcuate arm 42 of brass. having afoot 44. at one end thereof which is weldedtolhe end of radial, arm 12. The outer portion of thev bi-metallic arm 38. comprises. a. nonrmagnetic. metal having aco-eflicient of thermal expansion considerably greater than, that of the brass inner portion. 42. Aluminum is satisfactory for this purpose. Poising screws 54 are mounted. in the end of the composite arcuate arm 38 to provideapoising and. compensating adjustment. Composite arcuate arm 40 is-similarly formed of an inner brass arcuate member 46having a foot 48.welded to the end of radial arm 18. An outer arcuate member 50 of aluminum completes this bi-metallic arm. Poisingvscrews 52 similar to screws 54 are mounted in the end of the arm.

The balance wheel thus formed is completely nonmagnetic and may be-combined with anon-magnetic balance spring to form a temperature compensated frequency regulating element. By reason of the coil mounted on the balance wheel it may also be used to drive the train of an electric watch' This wheel provides economical and efficient mounting for the coil with a convenient arrangement for counterbalancing and poising the wheel after the coil has been added. The balance wheel is temperature compensated and may be used with a non-magnetic spiral spring whose characteristics vary with temperature.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by the United States Letters Patent is:

1. A temperature compensated non-magnetic balance wheel for an electric watch comprising, a hub, four arms 4. extending outward from said hub, a coil mounted between two of said'arms, and a pair of arcuate bi-metallic members connected to diametrically opposite arms at the outer extremities thereof, said bi-metallic members being formed of non-magnetic material.

2. A temperature compensated non-magnetic balance wheel for an electric watch comprising, a hub, four arms extending outward from said hub, a pair of arcuate members connecting the adjacent ends of oppositely disposed arms, a. coil mounted. between two of said arms and the arcuate member connecting them together, a synthetic resin member mounted between the arms and arcuate member opposite said arms and arcuate member enclosing said coil, a first arcuate bi-metallic member connected to the extremityof one of said arms, and a second arcuate bi-metallic member connected to the extremity of the arm opposite to said last named arm, said bimetallic members being formed of non-magnetic material.

3. A balance wheel asset out in claim 2 wherein said four arms are radial and said coil is sector shaped.

4. A balance wheel as set out in claim 3 wherein said synthetic resin member has poising weights mounted therein radially inwardly of the arcuate member which encloses it.

5. A balance wheel as set out in claim 4 wherein said hub, radial, arms and arcuate members are formed of beryllium copper.

6. A balance wheel as set out in claim 4 wherein said bi-metallic arcuate arms are welded to said radial arms.

References Cited in the fileof this patent UNITED STATES PATENTS 359,093 Paillard Mar. 8, 1887 487,550 Carpenter Dec. 6, 1892 489,790 Carpenter Jan. 10, 1893 2,239,668 Aegler, Apr. 29, 1941 FOREIGN PATENTS 764,450 Great Britain Dec. 28, 1956 

